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Abstract We present Keck Cosmic Web Imager integral-field unit observations around extended Lyαhalos of 27 typical star-forming galaxies with redshifts 2.0 <z< 3.2 drawn from the MOSFIRE Deep Evolution Field survey. We examine the average Lyαsurface brightness profiles in bins of star formation rate (SFR), stellar mass (M_*), age, stellar continuum reddening, SFR surface density (Σ_SFR), and Σ_SFRnormalized by stellar mass (Σ_sSFR). The scale lengths of the halos correlate with stellar mass, age, and stellar continuum reddening and anticorrelate with SFR, Σ_SFR, and Σ_sSFR. These results are consistent with a scenario in which the down-the-barrel fraction of Lyαemission is modulated by the low-column-density channels in the interstellar medium, and in which the neutral gas covering fraction is related to the physical properties of the galaxies. Specifically, we find that this covering fraction increases with stellar mass, age, andE(B−V) and decreases with SFR, Σ_SFR, and Σ_sSFR. We also find that the resonantly scattered Lyαemission suffers greater attenuation than the (nonresonant) stellar continuum emission, and that the difference in attenuation increases with stellar mass, age, and stellar continuum reddening, and decreases with Σ_sSFR. These results imply that more reddened galaxies have more dust in their circumgalactic medium. 

We investigate the multi-phase structure of gas flows in galaxies. We study 80 galaxies during the epoch of peak star formation (1.4 ≤ z ≤ 2.7) using data from Keck/LRIS and VLT/KMOS. Our analysis provides a simultaneous probe of outflows using UV emission and absorption features and Hα emission. With this unprecedented data set, we examine the properties of gas flows estimated from LRIS and KMOS in relation to other galaxy properties, such as star formation rate (SFR), star formation rate surface density (ΣSFR), stellar mass (M∗), and main sequence offset (ΔMS). We find no strong correlations between outflow velocity measured from rest-UV lines and galaxy properties. However, we find that galaxies with detected outflows show higher averages in SFR, ΣSFR, and ΔMS than those lacking outflow detections, indicating a connection between outflow and galaxy properties. Furthermore, we find a lower average outflow velocity than previously reported, suggesting greater absorption at the systemic redshift of the galaxy. Finally, we detect outflows in 49% of our LRIS sample and 30% in the KMOS sample, and find no significant correlation between outflow detection and inclination. These results may indicate that outflows are not collimated and that Hα outflows have a lower covering fraction than low-ionization interstellar absorption lines. Additionally, these tracers may be sensitive to different physical scales of outflow activity. A larger sample size with a wider dynamic range in galaxy properties is needed to further test this picture. 

Abstract Using deep near-infrared Keck/MOSFIRE observations, we analyze the rest-optical spectra of eight star-forming galaxies in the COSMOS and GOODS-N fields. We reach integration times of ∼10 hr in the deepest bands, pushing the limits on current ground-based observational capabilities. The targets fall into two redshift bins, of five galaxies atz∼ 1.7 and three galaxies atz∼ 2.5, and were selected as likely to yield significant auroral-line detections. Even with long integration times, detection of the auroral lines remains challenging. We stack the spectra together into subsets based on redshift, improving the signal-to-noise ratio on the [Oiii]λ4364 auroral emission line and, in turn, enabling a direct measurement of the oxygen abundance for each stack. We compare these measurements to commonly employed strong-line ratios alongside measurements from the literature. We find that the stacks fall within the distribution ofz> 1 literature measurements, but a larger sample size is needed to robustly constrain the relationships between strong-line ratios and oxygen abundance at high redshift. We additionally report detections of [Oi]λ6302 for nine individual galaxies and composite spectra of 21 targets in the MOSFIRE pointings. We plot their line ratios on the [Oiii]λ5008/Hβversus [Oi]λ6302/Hαdiagnostic diagram, comparing our targets to local galaxies and Hiiregions. We find that the [Oi]/Hαratios in our sample of galaxies are consistent with being produced in gas ionized byα-enhanced massive stars, as has been previously inferred for rapidly forming galaxies at early cosmic times. 

ABSTRACT The connection between the escape fraction of ionizing radiation (fesc) and the properties of galaxies, such as stellar mass ($$\rm M_{\rm *}$$), age, star-formation rate (SFR), and dust content, are key inputs for reionization models, but many of these relationships remain untested at high redshift. We present an analysis of a sample of 96 $$z$$ ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey (KLCS). These galaxies have both sensitive Keck/LRIS spectroscopic measurements of the Lyman continuum (LyC) region, and multiband photometry that places constraints on stellar population parameters. We construct composite spectra from subsamples binned as a function of galaxy property and quantify the ionizing-photon escape for each composite. We find a significant anti-correlation between fesc and $$\rm M_{\rm *}$$, consistent with predictions from cosmological zoom-in simulations. We also find significant anti-correlation between fesc and E(B−V), encoding the underlying physics of LyC escape in our sample. We also find no significant correlation between fesc and either stellar age or specific SFR (= SFR/$$\rm M_{\rm *}$$), challenging interpretations that synchronize recent star formation and favorable conditions for ionizing escape. The galaxy properties now shown to correlate with fesc in the KLCS are Lyα equivalent width, UV Luminosity, $$\rm M_{\rm *}$$, SFR, and E(B−V), but not age or sSFR. This comprehensive analysis of galaxy properties and LyC escape at high redshift will be used to guide future models and observations of the reionization epoch. 

ABSTRACT The connection between the escape fraction of ionizing photons (fesc) and star formation rate surface density (ΣSFR) is a key input for reionization models, but remains untested at high redshift. We analyse 35 z ∼ 3 galaxies from the Keck Lyman Continuum Survey (KLCS) covered by deep, rest far-UV spectra of the Lyman continuum (LyC) and high-resolution HST V606 imaging, enabling estimates of both fesc and rest-UV sizes. Using Sérsic profile fits to HST images and spectral-energy distribution fits to multiband photometry, we measure effective sizes and SFRs for the galaxies in our sample, and separate the sample into two bins of ΣSFR. Based on composite spectra, we estimate 〈fesc〉 for both ΣSFR subsamples, finding no significant difference in 〈fesc〉 between the two. To test the representativeness of the KLCS HST sample and robustness of this result, we attempt to recover the well-established correlation between fesc and Lyα equivalent width. This correlation is not significant within the KLCS HST sample, indicating that the sample is insufficient for correlating fesc and galaxy properties such as ΣSFR. We perform stacking simulations using the KLCS parent sample to determine the optimal sample size for robust probes of the fesc-ΣSFR connection to inform future observing programs. For a program with a selection independent of ionizing properties, ≥90 objects are required; for one preferentially observing strongly-leaking LyC sources, ≥58 objects are required. More generally, measuring the connection between fesc and ΣSFR requires a larger, representative sample spanning a wide dynamic range in galaxies properties such as ΣSFR. 

Aims. We aim to quantify the relation between the dust-to-gas mass ratio (DTG) and gas-phase metallicity of z  = 2.1 − 2.5 luminous galaxies and contrast this high-redshift relation against analogous constraints at z  = 0. Methods. We present a sample of ten star-forming main-sequence galaxies in the redshift range 2.1 <  z  < 2.5 with rest-optical emission-line information available from the MOSDEF survey and with ALMA 1.2 millimetre and CO J  = 3 − 2 follow-up observations. The galaxies have stellar masses ranging from 10 10.3 to 10 10.6   M ⊙ and cover a range in star-formation rate from 35 to 145 M ⊙ yr −1 . We calculated the gas-phase oxygen abundance of these galaxies from rest-optical nebular emission lines (8.4 < 12 + log(O/H) < 8.8, corresponding to 0.5−1.25 Z ⊙ ). We estimated the dust and H 2 masses of the galaxies (using a metallicity-dependent CO-to-H 2 conversion factor) from the 1.2 mm and CO J  = 3 − 2 observations, respectively, from which we estimated a DTG. Results. We find that the galaxies in this sample follow the trends already observed between CO line luminosity and dust-continuum luminosity from z  = 0 to z  = 3, extending such trends to fainter galaxies at 2.1 <  z  < 2.5 than observed to date. We find no second-order metallicity dependence in the CO – dust-continuum luminosity relation for the galaxies presented in this work. The DTGs of main-sequence galaxies at 2.1 <  z  < 2.5 are consistent with an increase in the DTG with gas-phase metallicity. The metallicity dependence of the DTG is driven by the metallicity dependence of the CO-to-H 2 conversion factor. Galaxies at z  = 2.1 − 2.5 are furthermore consistent with the DTG-metallicity relation found at z  = 0 (i.e. with no significant evolution), providing relevant constraints for galaxy formation models. These results furthermore imply that the metallicity of galaxies should be taken into account when estimating cold-gas masses from dust-continuum emission, which is especially relevant when studying metal-poor low-mass or high-redshift galaxies. 

Abstract We investigate the effects of stellar populations and sizes on Lyαescape in 27 spectroscopically confirmed and 35 photometric Lyαemitters (LAEs) atz≈ 2.65 in seven fields of the Boötes region of the NOAO Deep Wide-Field Survey. We use deep HST/WFC3 imaging to supplement ground-based observations and infer key galaxy properties. Compared to typical star-forming galaxies (SFGs) at similar redshifts, the LAEs are less massive (M_⋆≈ 10⁷–10⁹M_⊙), younger (ages ≲1 Gyr), smaller (r_e< 1 kpc), and less dust-attenuated (E(B−V) ≤ 0.26 mag) but have comparable star formation rates (SFRs ≈ 1–100M_⊙yr⁻¹). Some of the LAEs in the sample may be very young galaxies having low nebular metallicities (Z_neb≲ 0.2Z_⊙) and/or high ionization parameters ( $\log \left(\mathrm{U}\right)\gtrsim -2.4$). Motivated by previous studies, we examine the effects of the concentration of star formation and gravitational potential on Lyαescape by computing SFR surface density, Σ_SFR, and specific SFR surface density, Σ_sSFR. For a given Σ_SFR, the Lyαescape fraction is higher for LAEs with lower stellar masses. The LAEs have a higher Σ_sSFR, on average, compared to SFGs. Our results suggest that compact star formation in a low gravitational potential yields conditions amenable to the escape of Lyαphotons. These results have important implications for the physics of Lyαradiative transfer and for the type of galaxies that may contribute significantly to cosmic reionization. 

Abstract We use the large spectroscopic data set of the MOSFIRE Deep Evolution Field survey to investigate some of the key factors responsible for the elevated ionization parameters (U) inferred for high-redshift galaxies, focusing in particular on the role of star-formation-rate surface density (Σ_SFR). Using a sample of 317 galaxies with spectroscopic redshiftsz_spec≃ 1.9–3.7, we construct composite rest-frame optical spectra in bins of Σ_SFRand infer electron densities,n_e, using the ratio of the [Oii]λλ3727, 3730 doublet. Our analysis suggests a significant (≃3σ) correlation betweenn_eand Σ_SFR. We further find significant correlations betweenUand Σ_SFRfor composite spectra of a subsample of 113 galaxies, and for a smaller sample of 25 individual galaxies with inferences ofU. The increase inn_e—and possibly also the volume filling factor of dense clumps in Hiiregions—with Σ_SFRappear to be important factors in explaining the relationship betweenUand Σ_SFR. Further, the increase inn_eand SFR with redshift at a fixed stellar mass can account for most of the redshift evolution ofU. These results suggest that the gas density, which setsn_eand the overall level of star formation activity, may play a more important role than metallicity evolution in explaining the elevated ionization parameters of high-redshift galaxies. 

Abstract We present observations of CO(3−2) in 13 main-sequence z = 2.0–2.5 star-forming galaxies at log ( M * / M ⊙ ) = 10.2 – 10.6 that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that L CO ( 3 − 2 ) ′ /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z ∼ 2. We constrain the CO-to-H 2 conversion factor ( α CO ) and find that α CO inversely correlates with metallicity at z ∼ 2. We derive molecular gas masses ( M mol ) and characterize the relations among M * , SFR, M mol , and metallicity. At z ∼ 2, M mol increases and the molecular gas fraction ( M mol / M * ) decreases with increasing M * , with a significant secondary dependence on SFR. Galaxies at z ∼ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR− M * , O/H− M * , and M mol − M * relations is correlated such that, at fixed M * , z ∼ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ∼ 2, where O/H is inversely correlated with both SFR and M mol at fixed M * . These results suggest that the scatter of the z ∼ 2 star-forming main sequence, mass–metallicity relation, and M mol – M * relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z ∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium.